A typical gas turbine includes an inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section progressively increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The compressed working fluid and a fuel are mixed within the combustion section and burned in a combustion chamber to generate combustion gases having a high temperature and pressure. The combustion gases are routed along through a hot gas path into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion section generally includes one or more combustors annularly arranged and disposed between the compressor section and the turbine section. Various parameters influence the design and operation of the combustors. For example, gas turbine manufacturers are regularly tasked to increase gas turbine efficiency without producing undesirable air polluting emissions. The primary air polluting emissions typically produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs). Oxidation of molecular nitrogen and thus the formation of NOx in air breathing engines such as gas turbines is an exponential function of temperature. The higher the temperature of the combustion gases, the higher the rate of formation of the undesirable NOx emissions.
One way to lower the temperature of the combustion gases, thus controlling the formation of NOx, is to pre-mix fuel and air using a fuel injector or fuel nozzle that includes a plurality of swirler vanes disposed in a pre-mix flow passage and a plurality of fuel injection ports disposed upstream from or along an outer surface of the swirler vanes to create a lean combustible mixture in a pre-mix chamber of the combustor prior to injection into the combustion chamber. During combustion, the heat capacity or thermal capacitance of the excess air present in the air rich or lean combustible mixture absorbs heat in the combustion chamber, thus reducing the temperature of the combustion gases, thereby decreasing or preventing the formation of NOx emissions.
A flashback or flame holding condition may occur in combustors having pre-mix chambers for various reasons. Flashback typically occurs when flame propagates upstream from the combustion chamber into the pre-mix chamber, typically caused by momentary transient conditions. Flame holding typically occurs when a flame is initiated in the pre-mixing chamber. The flame then stabilizes in a recirculation zone or weak boundary layer zone formed immediately downstream of a portion of the swirler assembly where fuel is discharged into the pre-mix chamber. For example, the recirculation zone may be formed due to flow disturbances caused in part by the fuel pegs.
In some combustors, a non-symmetric flow in the vicinity of an injection point where the lean combustible mixture enters the combustion chamber plays a key factor in promoting flame holding. As a result, the flow field of the lean combustible mixture exiting the pre-mixer and entering the combustion chamber at the injection point should be uniform or symmetric in order to reduce the potential for flame holding and to achieve desired emissions performance.
Flashback and/or flame holding conditions within the combustor may result in undesirable thermal stresses on the fuel nozzles, thereby adversely affecting the mechanical life of the fuel nozzles, the swirlers and/or the combustor. Accordingly, an improved fuel nozzle that reduces flashback and/or flame holding within a combustor would be useful.